1. Field of the Invention
The present invention relates to a secondary winding excitation power converter for suppressing excess current generated in secondary windings of a doubly-fed generator by power system disturbance from flowing into the power converter.
2. Description of the Related Art
A doubly-fed generator (winding type induction generator) used in a power generator system has the following advantages. AC voltage having the same frequency as a system frequency can be output to a stator side by making a power converter excite the rotor windings at a slip frequency, the number of rotations can be made variable, and a capacity of the power converter can be reduced more than that of the generator.
When voltage lowers by ground fault of the power system, the doubly-fed generator operates to supply current to the ground fault point. In this case, excess current is induced in the secondary windings and flows into an excitation power converter connected to the secondary side. In order to resist against excess current, an element capacity of the power converter is set to the same degree as that of the generator ratings or a higher degree, or as disclosed in JP-A-HEI-11-18486 (Paragraphs [0010] to [0012]), excess current is absorbed by short-circuiting the secondary windings with a switching element and a resistor connected to the switching element.
With the conventional method of increasing the capacity of the power converter, however, a system cost increases, degrading the system feature using a doubly-fed generator. With the method of JP-A-HEI-11-18486, after the power converter is stopped once and excess current is removed, the short-circuit is released to activate again the system. Therefore, it takes time to supply power again.